This paper presents a method to model a human- exoskeleton interaction for patients suffering from cerebral palsy (CP). More precisely a model of the gait related to spastic CP is proposed using an optimization program based on experimental data. The model is done using mechanical differential equations of motion. A unique feature of this paper is the Clinical Gait Analysis (CGA) performed on two 9 years old twin sisters. One has spastic cerebral palsy (C) while the other is healthy (H) thus without any impairment. This paper aims at determining the proportion of the walking efforts that can be supported by the exoskeleton in order to allow a CP child gait to converge toward a non-pathological one. For this purpose, minimal torques produced by the human in interaction with the exoskeleton were studied. The interaction between the human and the exoskeleton is realized using optimisation methods such as SLSQP and QuadProg. Ground contacts are also included in the modelisation. Results show that the human produces joint torques in the same range of the ones of C. Exoskeleton succeds in producing additionnal torques to lead the pathological gait to a non-pathological one. The code for running the simulations is available on git.